In the determination of phosphorus in seawater, removal of arsenic was not considered as significant in some of the earlier methods. Strickland and Parsons recommended in their method for total phosphorus the use of potassium iodide to remove arsenic by volatilisation with perchloric acid. In the present method, sodium sulphite is used for the same purpose. Further, stannous chloride which is still widely used as a reducing agent for developing final phosphomolybdate blue colour, suffers from a number of disadvantages. The present method makes use of mixed reagent used by Portmann and Riley. The mixed reagent contains ascorbic acid as reducing agent. Confirmation of complete removal of arsenic and non-interference by silica under the conditions of the experiment was obtained. Values of phosphorus content of seawater samples collected from Bombay Harbour Bay are also presented.